fix(failover): patch P1-P3 reliability gaps surfaced by team review

- Forwarder buffers non-streaming bodies and primes streaming first
  chunk before signaling success, so body timeouts and SSE first-chunk
  failures route through the circuit breaker instead of being recorded
  as success on response-header arrival
- Atomic enable-failover: switch to P1 before persisting the flag, and
  roll back auto-added queue entries when the switch is rejected
  (e.g. official providers)
- Hot-reload circuit breaker config on per-app proxy config change
  instead of waiting for a proxy restart
- FailoverToggle / FailoverQueueManager / AutoFailoverConfigPanel
  require proxy takeover for the active app; the backend command also
  rejects enabling when takeover is off
- ProviderHealthBadge consumes the backend is_healthy flag instead of
  hardcoding the 5-failure threshold

Cleanup:
- impl From<&AppProxyConfig> for CircuitBreakerConfig and use it from
  the command layer
- Collapse three identical TabsContent blocks into a single map

src-tauri/src/services/proxy.rs

@@ -1993,6 +1993,23 @@ impl ProxyService {
         Ok(())
     }
 
+    /// 热更新指定应用的熔断器配置
+    pub async fn update_circuit_breaker_config_for_app(
+        &self,
+        app_type: &str,
+        config: crate::proxy::CircuitBreakerConfig,
+    ) -> Result<(), String> {
+        if let Some(server) = self.server.read().await.as_ref() {
+            server
+                .update_circuit_breaker_config_for_app(app_type, config)
+                .await;
+            log::info!("已热更新 {app_type} 运行中的熔断器配置");
+        } else {
+            log::debug!("{app_type} 熔断器配置将在下次代理启动时生效");
+        }
+        Ok(())
+    }
+
     /// 重置指定 Provider 的熔断器
     ///
     /// 如果代理服务器正在运行，立即重置内存中的熔断器状态